Since the 1930's when U.S. Pat. No. 1,927,664 was issued to Karcher, problems, described in the prior art, were associated with both the “mud pulse” fluid and the acoustic means of transferring information. These problems have, to a substantial extent, been solved most in most shallow well applications by using higher reliability electromagnetic (“EM”) means by which electrical energy radiates through the surrounding soil formation up to the surface. However, as wells become deeper and also when the formation being drilled through becomes more conductive, the traditional EM means will eventually no longer be effective in radiating sufficiently to reach the surface if relying on passage through the formation—because the EM energy dissipates in the formation to a level below the detection threshold at the surface. Although all EM means traditionally involve an uphole transmitter to radiate into the formation near the surface, the EM solutions to the “deep well” and high conductivity formation problems may be grouped into three (3) quite different categories of teaching:
1) Insulated cable extensions between downhole equipment and uphole transmitter;
2) Multiple Radiating Gaps (MRG) one originating, all transmitting; and
3) Externally Mounted Repeating Transmitters (ExMRTx)
The insulated cable approach has two primary disadvantages:                a) uses an expensive and fragile conducting cable; and        b) requires significant time to install, recover, and periodically replace        
The MRG approach has two primary disadvantages:                a) high power consumption (short battery life) passing sufficient current across the gaps in order to radiate sufficient energy from those gaps; and        b) sensitivity to the composition of the formation between the borehole and the surface sensing point (electrode).        
And, the ExMRTx suffer four primary disadvantages:                a) high power consumption (short battery life) passing sufficient current across the gaps in order to radiate sufficient energy from those gaps; and        b) sensitivity to the composition of the geologic formation between the borehole and the surface sensing point (electrode);        c) high noise sensitivity demanding more complex electronics and signal processing; and        d) inability to deploy in exploration mode (i.e. only applies to “operational” wells).        
Therefore, a need has arisen for an economical system that is capable of more effectively using the traditional component spaces available in a drill-string. The objects of the present invention include:                a) eliminate fragile conducting cables;        b) lower power consumption to extend battery life;        c) desensitize method to formation composition “away from” the borehole;        d) desensitize apparatus to noise permitting the use of more robust, simpler electronics and signal processing; and        e) simplify installation in any well functional (exploration or operational) mode.        
The fundamental energy transport mechanisms for heat, mass, and momentum (all forms of energy) are: conduction, convection, and radiation, all 3 of which mechanisms are always involved in the actual transfer of energy—however, different mechanisms dominate in different environments. In the deep-well downhole environment both convection and radiation have limited influence, as they would in a submarine environment. In fact in studies conducted by the US Navy using extremely low frequency (“ELF”)—conventional “radio” techniques based on electro-magnetic radiation have been determined to be impractical in electrically conductive sea water. This is significant because the prior art reviewed fails to address the electrical characteristics of drilling mud, which has some similarity to sea water. Moist soil will also bear some similarity to sea water, such that the combination of drilling mud and moist soil as a communication medium for downhole data transfer suggests the ELF range analysis will be a useful contribution to the cumulative wisdom of electric field telemetry (EFT) in this industry.
The US Navy has also determined that generating a “useful signal” using the traditional radiating antenna model of EM communications requires an unusually long physical antenna because the antenna length is inversely proportional to the frequency. By example, to achieve any “reasonable efficiency” at a frequency of 76 Hz, the Navy constructed two antennas each made up of two or three parallel power lines—each line being at least 14 miles long. Since most EFT prior art patents teach operation in the 2–10 Hz range, the above suggests that describing the average drill-string or any of its components as an “antenna” is likely not appropriate and possibly misleading. The inventor does not accept the descriptions provided in the prior art despite having to use those descriptions in reviewing what said art teaches.
Electric current is conventionally defined as the rate of flow of positive charge despite the fact that when using metal conductors (including drill-pipe segments), the mobile charge element is actually the negatively charged electron. As electrons move from one location on a metal surface to another location, they leave behind a transient void of negative charge that appears as a brief but relatively positive state. Due to the high mobility of electrons on a metal surface the void is quickly filled by electrons from an adjacent region of the drill-pipe surface, which process repeats indefinitely until the metal surface equilibrium state is restored from an external source (possibly mobile charge in the drilling mud) or is disturbed otherwise. In a drilling environment the path of least resistance is going to be the metal drill-pipe along which such disturbances will ripple, with each disturbance forming a wave-front. If drill-pipe were made of a material (e.g. ceramic) that does not support highly mobile surface charge, then the present invention would not function—even though the prior art based on insulating gaps (with a conductive material acting as an electrode at each end) would still “radiate” into the formation.
Notwithstanding that all electric “fields” in theory extend to infinity in 3 dimensions, the practical presence of a field depends upon the measurable effect it can have on relevant charged bodies in its vicinity. It is common knowledge that an accelerating charge “radiates” energy in the form of an electromagnetic field that propagates outward disturbing all (electric, magnetic, and electro-magnetic) fields (both static and dynamic) present in the space through which it passes. When the same charge reaches a steady state (whether stationary or moving with constant velocity) it ceases to “radiate”. Consequently, in the ELF range, the relatively low acceleration of (long wavelength) charge in the current flowing across insulating gaps results in low levels of radiation. Instead, as the charge configuration creating the baseline electric field around the surface electrode is disturbed by this influence propagating through the formation (via displacement effect) each time a “pulse” crosses a gap—a temporally retarded potential difference is transiently generated in that space—creating a disturbance in the baseline or equilibrium potential difference in the earth between the blowout preventer and the surface sensing electrode. This is the potential difference the disturbance of which is detected by the prior art (e.g. U.S. Pat. No. 4,468,665, discussed below), and which, because it does not depend on the pipe surface relaxing, has the greater potential bandwidth required in only some applications. This field disturbance will be superimposed on the fields resulting from the surface charge mobility, but the influence that it has on those fields will depend on the formation composition. The above described “electrical” effect is similar in all of the EFT prior art patents reviewed.
The conventional deep-well EM data transfer system, such as that disclosed in U.S. Pat. Nos. 6,188,223 and 5,883,516, FIG. 2b thereof, discussed below, consists of:                a) a drill head sensor and encoding (typically using a Binary Phase Shift Key “BPSK” scheme) electronics package;        b) a downhole power source (typically a lithium ion battery)        c) a downhole amplifier and coupling means to transfer current across a downhole insulating gap into the formation;        d) a lengthy and expensive insulated conductor.        
In order to overcome the disadvantages of the above prior art approach in an efficient manner it was necessary to first do two things:                1) understand the electro-physical principles involved in surface detectable EM measurements; and        2) understand what information needs to be transferred uphole in the majority of the applications of EFT.        
An information-carrying energy flow may be efficiently channelled along a drill-string, despite any radiation into the formation as a secondary effect incidental to feedback across the insulating gaps that prevent a direct short between the source terminals. Such information carrying flow is akin to a wave-front guided by a power transmission line. With the said flow following a narrow cylindrical channel along the drill-string, the composition of the formation (surrounding the borehole horizontally, and between the drill head and the surface electrode vertically) is no longer relevant for information transfer purposes. Only in the shallow surface layer between an optional uphole transmitter and its surface electrode could the electrical characteristics of the formation have any influence over the transfer of information. In fact, deep layers of highly conductive material in the formation would tend to insulate the surface antenna/electrode from any noise generated by drill-string gaps deeper in the formation.
Analysis of the above could be conducted (according to Jordan and Balmain, LCCCN 68-16319) by considering a chain of “Hertzian dipoles” each having a slightly different amplitude such that “the adjacent charges do not completely cancel, and there is an accumulation of charge on the surface” of the conductor. This iterative analysis places an understanding of the circuit involved in a conductive drill-string within the reach of simple circuit concepts (based on Ohm's Law) that are cumulatively compatible with the displacement effect described below. The chain of dipoles model is also consistent with a cylindrical “antenna” that is broken down into a series of short segments each being a separate circuit that results in an incremental loss feeding into the next segment (circuit). However, the non-uniform transmission line model guiding spherical wave fronts is a more effective means of understanding the manner in which a drill-string can be useful transferring data. Realizing that a drill-pipe is a hollow, large diameter, conductive tube with a finite wall thickness, is the starting point for understanding that capacitance can exist between points on a continuous conductive surface, which is only one departure from the relatively thin, solid core, perfect conductor assumed in the prior art traditional analysis. Also, recognizing that a normal length drill-string would not radiate per se in the relevant frequency range, it is clear that the prior art must in fact be using the drill-pipe segments as electrodes, the current flow between which segments generates magnetic fields normal to the direction of that current flow. Therefore, as set out above, it is via a displacement mechanism that the magnetic field influence then propagates through the formation to influence the electric field at the surface, causing a detectable disturbance in the potential difference between the blowout preventer and an electrode driven into the ground nearby. Even in deep well environments, prior art such as U.S. Pat. No. 6,075,461 to Halliburton Energy Services Inc continues to teach the use of such EM disturbances that propagate through the formation triggering a series of repeaters mounted external to the drill-pipe.
The prior art does not directly address the “efficiency” of the so-called antenna or even the efficiency of the impedance matching between the pipe and the formation, but it does indirectly recognize the importance of this factor when it teaches the need to “drive” sufficient current into the formation by ensuring that the resistance of the electrical path through the gap material is substantially higher resistance than the formation path. Since all EFT systems use a form of sensitive galvanometer at the surface to detect (as a change in electrical potential at the formation surface) the influence of a source of moving electrical charge deep below the surface —it is clear that whatever propagates must have the capacity to disturb an electric field to a detectable extent.
Typical formations have dielectric characteristics, containing charged particles that have limited mobility. At the surface the charge in the formation will have reached a relatively stable state of equilibrium (as will the charge distributed throughout the inhomogeneous formation between the surface and the downhole source) that will experience a force via a displacement effect that transiently disturbs the equilibrium state each time charge pulses across the downhole gap. Starting from the basic premise that the further the point of detection (i.e. the pair of surface electrodes) is from the source (i.e. the gap) of the information carrying EM disturbance, the more charge must flow across that source gap to generate a specified level of detectable change in the static electric and magnetic fields at that point of detection. In any given formation, the higher the (charge flow per unit time) current, the stronger the field strength, and the deeper the well from which it can be detected, but the shorter the battery life. In a design that channels the displacement effect directly up the (highly conductive) metal drill-pipe, the attenuation takes place over a greater distance permitting detection over a longer range, requiring fewer repeaters and shorter bursts, also resulting in lower power consumption.
The prior art reviewed herein includes:                U.S. Pat. No. 6,188,223—Feb. 13, 2001 to Scientific Drilling International ('223)        U.S. Pat. No. 6,075,461—Jun. 13, 2000 to Halliburton Energy Services Inc ('461)        U.S. Pat. No. 5,942,990—Aug. 24, 1999 to Halliburton Energy Services Inc ('990)        U.S. Pat. No. 5,394,141—Feb. 28, 1995 to Geoservices ('141)        U.S. Pat. No. 4,468,665—Aug. 28, 1984 to Tele-Drill, Inc. ('665)        U.S. Pat. No. 4,087,781—May 2, 1978 to Raytheon Company ('781)        
None of the EFT prior art recognizes the electrode nature of the drill-pipe or offers a rigorous scientific analysis of the influence of the ionic solution (drilling mud) inside as well as surrounding the pipe and filling the annulus external to it. Clearly a moving ionic solution creates EM effects of its own, but its net influence on the EM fields resulting from the current flowing across the “gap” in the drill-string is left undefined and is therefore an opportunity to improve the teachings of the prior art in this field of invention.
Specifically, US '461 to a “Disposable Electromagnetic Signal Repeater” discloses an apparatus, system, and method for communicating real time information between surface equipment and downhole equipment using electromagnetic waves to carry the information. An electromagnetic signal repeater 34,36 is disclosed that may be securely mounted to the exterior of a pipe string 30 disposed in a well bore. A transmitter 44 generates electromagnetic waves that are picked up by a receiver of repeater 34, such repeater 34 mounted by straps on the exterior of the pipe string uphole from the transmitter 44. Repeater 34 is spaced along drill string 30 and above transmitter 44 to receive electromagnetic waves 46 while such waves 46 remain strong enough to be detected. The pipe string does not have any insulating (non-conductive) gaps. To prevent a direct electrical short circuit occurring between repeater 34 and tubing string 30 that would inhibit the propagation of electromagnetic waves 46, an insulating layer 108 is provided in the repeater 34,36. When repeater 34 re-transmits the electromagnetic waves that it has received, current flows through the lower part of the repeater 34 (housing subassembly 106) which is in electrical contact with pipe string 30, which current flow generates axial current in the pipe string 30 to produce electromagnetic waves 46 that propagate through the formation to an uphole repeater 48, which is capable of repeating the foregoing sequence.
Disadvantageously, to cause an axial current in the pipe string 30, electromagnetic signal repeaters 34,36 such as the type disclosed in US '461 (although not expressly so mentioned in US '461) typically utilize electromagnetic coils, which coils make repeaters 34,36 large, bulky, relatively expensive, and relatively high in power consumption (shortening their battery life). Further, due to such repeaters being mounted on the exterior of a pipe string, they are only suited to operational wells and not for MWD (“measurement while drilling”).
US '781 entitled “Electromagnetic Lithosphere Telemetry System” teaches repeater stations 144 spaced at predetermined intervals along the drilling pipe, and are contained in repeater sections 126 which form an integral part of drilling pipe assembly 125. Disadvantageously, solenoidal antenna 146 (ref. FIG. 3 thereof) comprised of high permeability core rods wrapped in wire coils, which in the preferred embodiment comprise a rod approximately 1 inch thick, 2 inches in width, and 20 feet in length, coupled at each end to a transceiver in the repeater station are required in order to transmit and receive the signals. The signals are transmitted via such antennae 146 through the formation (i.e. through the earth's lithosphere) to the next repeater station. Pipe strings without any insulating gaps are used.
US '223 entitled “Electric Field Borehole Telemetry” teaches the use of wave forms selected for “optimum transmission characteristics in the underground formation”. Further, FIG. 1b illustrates an assembly in which each battery and circuitry assembly has a single connection on each side of each gap. These two factors confirm that the information transmission path is through the formation, and not via the pipe string since the objective of each gap is merely to impose a barrier around which the current will prefer to pass through the “formation” for which US 223 teaches impedance has been optimized. Disadvantageously, the means of transmission requires an electrically conductive cable 6 extend down the center of the pipe string. This cable 6 breaks frequently during installation, use, and removal.
US '990 entitled “Electromagnetic Signal Repeater and Method for use of Same”, teaches an apparatus and method based on an interiorly mountable repeater, suited to MWD applications Insofar as could arguably be said to relate to the inventions later set out herein, US '990 teaches at col. 9 & 10, and FIGS. 4A&B, an electromagnetic signal repeater 330, having an isolation subassembly 348 which provides a discontinuity in the electrical connection between lower connector 352 and upper subassembly 346 of the repeater 330 thus providing a discontinuity in the electrical connection between the portion of drill string 30 below repeater 330 and the portion of drill string 30 above repeater 330. In operation, a receiver 374 is provided to receive an electromagnetic input signal (delivered via the earth and not the drill pipe since such signal propagates through the formation—see below) carrying information that is transformed into an electrical signal that is passed onto electronics package 376 (sic-not identified) via electrical conductor 378. Electronics package 322 (sic-378?) processes and amplifies the electrical signal. An output voltage is then applied between intermediate housing member 342 and lower mandrel section 358, which is electrically isolated from intermediate housing member 342 and electrically connected to lower connector 352, via terminal 380 on intermediate housing member 342 and terminal 382 on lower mandrel section 358. The voltage applied between intermediate housing member 342 and lower connector 352 generates a current flow through the geologic formation proximate the repeater that results in an electromagnetic output signal that is “radiated” into the formation. Unlike the present invention, a significant and patentably distinct difference between US '990 and the present invention, as will later be more fully explained, is that the input signal to the repeater 330, and more particularly to the receiver 374 of US '990, is electromagnetic in nature and is received by the receiver 374 after and via its passage through the earth rather than along the drill pipe. Coil based designs such as that of the receiver of US '990 are very sensitive to noise resulting in the need to use both more expensive electronic components and more sophisticated signal processing in their implementation. Moreover, the signal distortion in schemes such as that of US '990, which amplify and repeat the subject signal, without a “silence time” delay, build in a cumulative error unlike the detection and replacement scheme inherent in a silence time based design.
US '141 entitled “Method and apparatus for transmitting information between equipment at the bottom of a drilling or production operation and the Surface”, as the title suggests, relates to methods of transmitting information from downhole equipment. Such patent teaches the use of insulated wires, which are problematic for the reasons given above
US '665 teaches a power amplifier used in this environment.